To create a custom collector in Java Streams, use Collector.of() with supplier, accumulator, combiner, and finisher. 1. Supplier initializes the container. 2. Accumulator adds elements to it. 3. Combiner merges containers, crucial for parallel streams. 4. Finisher converts the result to the desired type. Ensure combiner works correctly for parallelism. Use custom collectors when reuse or API integration is needed, otherwise prefer reduce(). Test thoroughly, especially for parallel use.
When you're working with Java Streams and need to do something more specific than what Collectors.toList() or Collectors.toSet() offer, creating a custom collector is the way to go. The key is implementing the Collector interface properly while understanding how each part fits into the stream pipeline.
What You Need Before Writing a Custom Collector
Before diving into writing your own collector, make sure you understand:
- How Java Streams work, especially terminal operations.
- The difference between mutable and immutable reductions.
- That a collector defines four operations: supplier, accumulator, combiner, and finisher.
You’ll also want to import java.util.stream.Collector and related classes before starting.
A good example use case might be collecting data into an immutable custom container or aggregating results in a special format.
Implementing the Collector Interface Step by Step
Java provides a Collector.of() method that makes it easier to define all necessary parts without needing to write a full class that implements Collector.
Here’s how you can structure it:
Collector<T, A, R> collector = Collector.of(
() -> new IntermediateContainer(), // Supplier
(container, element) -> container.add(element), // Accumulator
(container1, container2) -> container1.merge(container2), // Combiner
container -> container.getResult() // Finisher
);Let’s break those parts down:
- Supplier: Returns a new result container. This is where you start fresh for each reduction.
- Accumulator: Adds elements to the container as they come in from the stream.
- Combiner: Merges two containers — this is important for parallel streams.
- Finisher: Converts the final intermediate result into the desired output type.
Make sure your combiner works correctly in parallel scenarios unless you know the collector will only be used sequentially.
Example: Collecting Into a Custom Immutable List
Say you want to collect elements into a custom immutable list wrapper. Here's how you'd do it:
public class MyImmutableList<T> {
private final List<T> list;
public MyImmutableList(List<T> list) {
this.list = Collections.unmodifiableList(new ArrayList<>(list));
}
public static <T> Collector<T, ?, MyImmutableList<T>> toMyImmutableList() {
return Collector.of(
ArrayList::new,
List::add,
(list1, list2) -> {
list1.addAll(list2);
return list1;
},
MyImmutableList::new
);
}
}Then you can use it like this:
MyImmutableList<String> result = stream.collect(MyImmutableList.toMyImmutableList());
This pattern gives you flexibility and keeps your stream pipelines clean.
One thing to watch out for: if you forget to handle the combiner properly, your collector may not behave correctly in parallel streams.
When to Use Custom Collectors vs Regular Reductions
Custom collectors are best when:
- You find yourself repeating the same reduce logic across multiple places.
- You need to integrate with existing APIs that expect a
Collector. - You want to abstract away complex collection logic for cleaner code.
On the other hand, if it's a one-off operation, just using reduce() might be simpler and more readable.
So ask yourself: will I reuse this? If yes, build a collector. If no, stick with reduce or a simple mutable container.
That's basically it. It’s not overly complicated once you get the pattern down, but there are enough moving parts that you should test thoroughly — especially if you plan to use your collector in parallel streams.
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